Conformal Coating in Nitrogen EnvironmentWe operate a conformal coat machine using nitrogen. We are thinking of changing it back to air for cost reasons, what are the pros and cons. We are under the impression that nitrogen is the better choice for purity reasons, is that correct?

M.E.

Expert's Panel Responses

Nitrogen
use for conformal coating system:

Pros:

Nitrogen (N2) is used in
this process for the following reasons:
A. N2 is low in moisture and if Compressed Air is used and the
system is not maintained properly, moisture will be present and this will cause
issues in the curing of the conformal coating on the parts, such as poor
adhesion, moisture entrapment, etc.
B. Compressed air can contain oils if the filters are not
maintained, oil is a major problem with conformal coatings
C. Particle free as compared to compressed air

Cons:

Cost of N2 is higher
than compressed air, however there are issues, as stated above, that may be
more costly such as rework, field failures, etc.
A.
Not knowing the supply mode that the M.E. is using, here are
some considerations that may lower his costs:
i. N2 generation membrane system
ii. Small liquid dewars that can be filled at your location and not
transported back to the supplier for fill

Conformal
coatings are basically a resin dissolved in a solvent. Most conformal coatings
utilize a flammable solvent. Once the coating is applied, the gas flow (air or
N2) is needed to carry the solvent vapors from the machine/spray booth.

If air
is the gas used, then the air flow must be sufficient to keep the solvent vapor
concentration below its LFL (Lower Flammability Limit). For example, the air
flow for a spray booth that my employer uses is 7000 cfm. Using N2 as the sweep
gas does not require as much flow since there's no oxygen in the gas flow. Hence,
the LFL (also known as the LEL (Lower Explosive Limit) would not apply in a N2
flow.

In addition to evaluating what air flow would be required to maintain.

Since air is 80% nitrogen, I don't understand how purity of the gas is involved.
If the application was a wave solder unit, then N2 would prevent oxidized
solder connections. But conformal coating???

I
presume that you are using nitrogen to pressurize the pressure pot on the
coating system containing the coating and possibly power the air assist.
Presuming
that this is a selective coating system.

Providing
that you filter and dry a compressed air supply you should not see any
difference.
The
only exception to this rule could be if you are using an oxygen cross linking
coating, polyurethane with oxygen cross linking for example.

There
would only be a negative effect here if the coating remained unused in the pot
for a significant amount of time, weeks or months, this is not good general
practice.
The
only other use of nitrogen in a coating system would be to form a blanket over
a dip coating tank to reduce evaporation and solvent loss.

Chris PalinEuropean ManagerHumiSealChris Palin is currently managing European sales and support for HumiSeal Conformal Coatings. His expertise is in test & reliability, solder technology, power die attach and conformal coating.

Without very specific knowledge of your process, including the coating materials involved, my comments must be rather general. The main benefit of nitrogen here is that it is very dry. By contrast, many factory compressed air sources contain a lot of moisture. Poor system design or maintenance can even result in water and/or oil in the lines.

If your specific process requires controlling the amount of moisture in the process, it may be as expensive to install and maintain drying and filtration systems as it is to run nitrogen for critical processes. On the other hand, if your compressed air system has effective dryers and filtration, then secondary filtration and/or drying may not be expensive, or even necessary.

Fritz ByleProcess EngineerAstronauticsFritz's career in electronics manufacturing has included diverse engineering roles including PWB fabrication, thick film print & fire, SMT and wave/selective solder process engineering, and electronics materials development and marketing. Fritz's educational background is in mechanical engineering with an emphasis on materials science. Design of Experiments (DoE) techniques have been an area of independent study. Fritz has published over a dozen papers at various industry conferences.

Besides purity & preventing solvent loss, the main reasons for choosing nitrogen as a blanket is its inertness and to keep the system dry. Many conformal coatings can exhibit "blushing" if moisture is absorbed into the solvents and then allowed to cure. In addition, RTV silicones require a minimum amount of moisture to initiate the curing process and so must necessarily remain moisture-free prior to application.

Another con of the air is that many operators will use "shop" air instead of dry compressed cylinder air. This has the potential to introduce contaminants into the coating through entrained oils etc that should be removed through the separators normally found in an air compression system. Unfortunately, this is not always the case, generally due to lack of frequent PM on the compressor, ie blowing down the separator system.

If you must switch to air due to economics, please consider using a dedicated cylinder of what is called "5.0" compressed air for your system. It is designed for use in chromatograph systems and is extremely dry and otherwise contaminant-free.

Pierce PillonLaboratory Mgr.TechsprayPierce Pillon is the Laboratory Manager and lead formulations chemist at Techspray, a division of Illinois Tool Works (ITW) and a leading manufacturer of chemical products for the electronics industry.

In general we have not seen many conformal coat applications that benefit from nitrogen.

So, in general, it is probably without risk to stop using nitrogen.

But let's throw in the following caveats and diligence:

Check with the vendor of the conformal coat material. They are really the bottom line on what type of atmosphere is best for their material. If they say ok to use air, that carries 1000 times more weight than what any other "expert" might be telling you.

Try a few samples and test the results. Data rules! So before you turn off the main Nitrogen valve on the production line try a few sample batches and have your customer or your quality people confirm that this still provides a satisfactory result.

Re-confirm the thermal profile of the curing oven. Some ovens have different thermal performance when running nitrogen vs. air. So it is a good idea to run a profile board just to make sure that you are getting the same temperature at board level with and without the nitrogen running.

Marc PeoPresidentHeller Industries Inc.Mr. Peo has been with Heller Industries for over 20 years and has been President for the past 8 years. Marc has authored several industry articles on Soldering, Flux collection, nitrogen use and Lead Free conversion.

You may use air as you have a good filtering and drying system.

Edithel MariettiSenior Manufacturing EngineeriDirectEdithel is a chemical engineer with 20 year experience in manufacturing & process development for electronic contract manufacturers in US as well as some major OEM's. Involved in SMT, Reflow, Wave and other assembly operations entailing conformal coating and robotics.

Reader Comment

Nitrogen is typically used to pressurize conformal coating
tanks when the material is moisture-sensitive and the factory air supply is not
dry enough. The level of sensitivity to moisture varies by material and the
moisture level found in house air will vary from factory to factory. With
moisture-sensitive fluids, using moist air can cause buildup inside the fluid
supply lines. The build-up causes flow to decrease and introduces gelled
material particles that may enter the fluid stream and clog the applicator. The
more sensitive the material is to moisture, the more significant the
consequences.

Bottled nitrogen is typically very dry and thus offers a
quick fix in these situations. You can add dryers to the air, however, that is
typically not as effective as using the dry nitrogen. An alternative solution
is the use of "bladder-bags" - these are air-tight bags in which the fluid
supplier packages coating materials, eliminating direct contact with
potentially moist air. You would need to request this packaging from your fluid
supplier and be sure your conformal coating system will handle the bags.

Nordson ASYMTEK Conformal Coating Applications Team

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